Several other methods produce a multi-layered model from a single casting procedure by placing a physical barrier into the negative impression prior to the casting of stone thereinto.
For example, in U.S. Pat. No. 3,470,614, a somewhat complicated pin and parting plate assembly is employed. In U.S. Pat. No. 3,153,283, a precise fit between a pin and sleeve is required.
In the above patents, and others, the parting plate, through the use of retentive media associated therewith, such as projections, grooves, and the like, render the parting plate an inseparable part of the final model.
The present invention discloses a dental model wherein edentulous portions thereof are detachable therefrom and vertically adjustable therein. The parting plate is discardable and may be replaced with a spacer of any desired thickness. The height of the edentulous portion of the dental model may therefore be raised, or lowered, with respect to the remainder of the teeth in the model, and precisely by the amount of difference between the thickness of the parting plate and selected spacer.
While an edentulous portion constructed to render it detachable from a model may be raised by the simple expediency of placing a shim thereunder, no present dental model making system is believed to teach methods or apparatus for its lowering. Advantages to be gained from edentulous portions capable of being lowered are several.
For example, pontic units on a fixed bridge should contact the tissues of edentulous areas to exert a very slight pressure thereupon in order to prevent food from being forced thereunder during mastication. Conversely, if the pontics exert excessive pressure, tissues become inflamed and unhealthy. Traditionally, dental casts were mechanically scraped or abraded in the edentulous portions in order that the pontics constructed thereupon would displace tissue proportionally to the amount abraded or removed from the dental cast or model.
Additionally, edentulous or ridge areas of a mouth are characterized by a complicated physiography comprising a series of depressions and raised areas which provide compound contiguous convexities and concavities. No method of uniformly scraping, abrading or removing such material is believed known or available. Consequently, many pontic units fit poorly. Further, many of these poorly fitting pontic units were made despite repeated visits to the dentist. An edentulous portion of a dental cast, which can be vertically adjusted, i.e., readily lowered as well as raised, obviates the need to meticulously scrape or abrade, which, as aforedescribed, often yet resulted in ill-fitting pontics.
Further, edentulous areas of recent origin are generally quite different from edentulous areas resulting from extractions of past years. More specifically, more tissue will be found over supporting bone substrate of recently formed edentulous areas. Thus, amount of tissue displacement presents another problem.
The present invention allows the dentist to prescribe an exact amount of tissue displacement for a given edentulous area which permits the technician to make more acceptable units.
To clarify more fully, most current fixed bridgework is of a type known as porcelain fused to metal. Porcelain powder is mixed with water to produce a paste-like slurry which forms the artificial teeth of a dental cast. When the shaping process is completed, the bridgework is removed from the model for firing in a kiln. During the firing process, the porcelain powder becomes a vitreous mass having a volume approximately 15% less than the original pre-fired mass. As a result of this shrinkage, a space develops under the pontic units of the bridge. In order to compensate, traditionally, a mass of porcelain slurry is added to the pontic units after the bridge has been removed therefrom but prior to its firing. The pontic will thus be oversized after firing requiring tedious grinding thereof in an attempt to mate the very complex surfaces of the edentulous portions of the dental cast.
In order to compensate for the porcelain shrinkage factor, the present invention, with its ability to provide for a lowering of the edentulous portion of the cast, additional porcelain may be built at the underside of the pontic. Thus, when the cast is brought back to its original configuration, only a minimal amount of adjustment by grinding will be necessary in order to insure a proper fit.